An imperfect vaccine reduces pathogen virulence

Leaky vaccines prevent the development of disease symptoms, but do not protect against infection and the onwards transmission of pathogens, which has been a concern because the consequences for unvaccinated contact individuals are unknown. Bailey et al. performed transmission experiments using Marek disease virus in chickens and found that the herpesvirus of turkeys vaccine significantly reduced feather viral load in both vaccinated birds and unvaccinated contact individuals. The authors found that contact birds were less likely to develop disease and die, and that they displayed milder symptoms and shed less virus, when infected by vaccinated birds, potentially because of a lower infectious dose. This study highlights that sterilizing immunity is not always needed to control the spread and severity of disease, and that leaky vaccines may have a role in disease management.